With the fast development of communication systems, computers and consumer electronics, the power supplies for telecoms, servers, desktops, laptops, flat-panel TVs, LED lighting, etc. are required for more efficient power delivery with smaller spaces. The LLC resonant converter has been widely adopted for these applications due to the advantages in high efficiency, high power density and holdup time operation capability. However, LLC resonant converter meets some issues, especially in high output current applications. Those issues include magnetic design, start-up, short-circuit protection, synchronous rectifier drive, EMI noise and integration, etc. To solve those issues, like start-up and short-circuit protection, SR driving and EMI, etc., a synthesis method is proposed to find the similar resonant topologies like LLC. Based on this method, lots of multi-element resonant converters are found to solve the issues that LLC resonant converter cannot handle. To evaluate the performance of found numerous valuable topologies. Thus, a general evaluation system is required. State-plane analysis with new normalization factors is utilized. Based on it, the voltage stress, current stresses and apparent power of resonant converters are easy to compare. This method can help select suitable circuit topology for certain applications. Meanwhile, it also can help resonant converters' design. The important performance factors, like start-up, short-circuit protection, SR driving, integration and EMI performance, are also taken into account for the whole evaluation system. The high switching frequency is needed recently for high power density requirement. However, LLC resonant converter suffers high transformer loss. Matrix transformer is introduced to reduce winding loss and total volume. Flux cancellation method is utilized to reduce core size and loss. Synchronous Rectifier (SR) devices and output capacitors are integrated into secondary windings to eliminate termination related winding losses, via loss and reduce leakage inductance. The passive integration is necessary for high power density resonant converter, especially for high order system. Based on stress, suitable passive components are chosen for integration. Then, the magnetic integration method is shown based on multi-winding transformer structure. The passive integration principles are discussed. A novel passive integration method is proposed for multi-elements resonant converters. In conclusion, this work is focus on the topology analysis and integration of resonant converters. Searching the suitable topologies for certain application, and evaluate the performance of them. Then, improve the system power density by integration techniques. / Ph. D.

With the fast development of communication systems, computers and consumer electronics, the power supplies for telecoms, servers, desktops, laptops, flat-panel TVs, LED lighting, etc. are required for more power delivery with smaller spaces. The LLC resonant converter has been widely adopted for these applications due to the advantages in high efficiency, high power density and holdup time operation capability. However, unlike PWM converters, the control of the LLC resonant converter is much more difficult because of the fast dynamic characteristic of the resonant tank. In some highly dynamic processes like the load transient, start-up, over-load protection and burst operation, it is hard to control the current and voltage stresses and oscillations in the resonant tank. Moreover, to meet the high power density requirement, the LLC is required to operate at a high switching frequency. Thus the driving of the synchronous rectifier (SR) poses a design challenge as well. To analyze the fast dynamic characteristic, a graphic state-plane technique has been adopted for a class of resonant converters. In this work, it has been extended to the LLC resonant converter. First of all, the LLC steady state and dynamic behaviors are analyzed in the state plane. After that, a simplified implementation of the optimal trajectory control is proposed to significantly improve the load transient response: the new steady state can be tracked in the minimal period of time. With the advantages of the state-trajectory analysis and digital control, the LLC soft start-up is optimized as well. The current and voltage stress is limited in the resonant tank during the start-up process. The output voltage is built up quickly and smoothly. Furthermore, the LLC burst mode is investigated and optimized in the state plane. Several optimal switching patterns are proposed to improve the light load efficiency and minimize the dynamic oscillations. During the burst on-time, the LLC can be controlled to track the steady state of the best efficiency load condition in one-pulse time. Thus, high light-load efficiency is accomplished. Finally, an intelligent SR driving scheme is proposed and its simple digital implementation is introduced. By sensing the SR drain to source voltage and detecting the paralleled body diode conduction, the SR gate driving signal can be tuned within all operating frequency regions. In conclusion, this work not only solves some major academic problems about analysis and control of the LLC resonant converter based on the graphic state plane, but also makes significant contributions to the industry by improving the LLC transient responses and overall efficiency. / Ph. D.

The demand for high power density, high efficiency bus converters has increased interest in resonant topologies, particularly the LLC resonant converter. LLC resonant converters offer several advantages in efficiency, power density, and hold up time extension capability. Among high voltage (>500V) MOSFETs, Super Junction MOSFETs, such as Infineon's CoolMOS parts, offer lower Rds on than conventional parts and are a natural choice for this application to improve efficiency. However, there is a history of converter failure due to reverse recovery problems with the primary switch's body diode. Before selecting CoolMOS devices for use in a LLC resonant converter, it is necessary to investigate its performance in this application. Field failures of PWM soft switching phase shift full bridge converters have been attributed to large reverse recovery charge in the primary side MOSFET body diode. Under low load conditions the device cannot fully recover, and the large reverse recovery current can cause the device to enter secondary break down, leading to failure. The unique structure of Super Junction MOSFETs, such as CoolMOS, avoid this failure mode by providing a different path for the reverse current; however, the reverse recovery charge of CoolMOS devices is large and can cause a loss of efficiency. For this reason, it is important to avoid conditions under which the reverse recovery characteristics of the body diode can be seen. / Master of Science

LLC resonant converters have been a popular choice for DC-DC converters due to their high efficiency, high power density, and hold-up capability in power supplies for communication systems, datacenters, consumer electronics, and automobiles. With the rapid development of wide-bandgap devices and novel magnetic materials, the push for higher switching frequencies to achieve higher power densities at lower costs is gaining traction. To demonstrate high efficiency and high power density, the Center for Power Electronics Systems (CPES) at Virginia Tech designed an 800W, 1MHz 400V/12V LLC converter for future datacenters, which could achieve a peak efficiency of 97.6% and a power density of 900 W/in3. However, with the ever-increasing demand for online services, the performance of power delivery must also be simultaneously improved to keep pace with the demand. The focus of this thesis is improving the performance of CPES’ previous 400V/12V LLC converter by investigating different aspects of its design and operation. Ultimately, design guidelines are proposed, and improvements are demonstrated to effectively achieve higher efficiency and higher power density than the previous CPES converter. Multiple aspects of the LLC converter’s design and structure are investigated to further improve its performance, and three main areas are the focus of this thesis. The output-side termination design of the planar transformer is investigated and modeled, and design guidelines for filter capacitor selection are provided for optimal efficiency. Next, the existing shielding technique for matrix transformers, which helps reduce common-mode (CM) noise without compromising on efficiency, is investigated for asymmetry and current-sharing issues, and modifications have been proposed to improve its efficiency. Thirdly, the LLC converter’s switching frequency is optimized to improve its performance over the previous CPES converter. Finally, the hardware results with the proposed improvements are demonstrated, and the converter’s performance is compared with the previous CPES converter as well as other recent proposed solutions. / M.S. / The electricity demand by datacenters has been growing exponentially over the past few decades, especially due to the boom of artificial intelligence in addition to other internet services. This has resulted in a requirement to continually improve the efficiencies of the power delivery from the grid, through the datacenter power architecture, and finally to the loads on the server racks. The overall datacenter power architecture has been improved over time to improve the total efficiency. However, the performance of each stage along the power architecture must be improved to keep in pace with the energy demand. The focus of this thesis is to improve the performance of the 400V/12V DC-DC stage for future datacenters. Previously, the Center for Power Electronics Systems (CPES) at Virginia Tech developed a 1MHz 800W 400V/12V LLC converter with 97.6% peak efficiency and 900W/in3 power density. However, the performance of the converter must be further improved to stay ahead of the competition and keep in pace with the increasing energy demand. Multiple aspects of the LLC converter’s design and structure are investigated to further improve its performance, and three main areas are the focus of this thesis. Firstly, the high-frequency termination design, or how different components are interconnected and arranged, is studied, and a capacitance selection guideline is proposed to maximize the efficiency. Next, the existing shielding technique for matrix transformers, which helps reduce common-mode (CM) noise without compromising on efficiency, is investigated for asymmetry and current-sharing issues, and modifications have been proposed to improve its efficiency. Thirdly, the LLC converter’s switching frequency is optimized to improve its performance over the previous CPES converter. Finally, the hardware results with the proposed improvements are demonstrated, and the converter’s performance is compared with the previous CPES converter as well as other recent proposed solutions.

This paper presents a single-stage inverter with variable frequency modulation (VFM) based on LLC resonant converter. And LLC converter is a common topology of dc/dc conversion. LLC resonant converter can achieve high efficiency and soft-switching performance. Since the dc gain curve of the single-resonant LLC converter is flat when the switching frequency is larger than the resonant frequency, namely fs>fr, an additional L-C series resonant branch is paralleled to the original resonant tank to introduce higher-order-harmonic resonant current and a zero-gain point to the gain curve. Higher-order-harmonics help to deliver power and the zero-gain point enlarges the gain range which improves output THD and reduces the switching frequency range. A 1.2 kW prototype is built to demonstrate the performance of the proposed inverter. Zero-voltage-switching (ZVS) and zero-current-switching (ZCS) are achieved on the primary side and secondary side, respectively. And 97.3% efficiency and 2.17% voltage THD are achieved at full load condition, while 97.2% efficiency and 3.2% voltage THD are achieved at half load condition. / M.S. / The inverter is widely used to connect renewable energy into the grid by converting dc to ac waveform, like photovoltaic (PV) technology. Basically, the two-stage topology is usually used. The inverter would consist of two stages working in high frequency, the first stage is dc/dc converter which can regulate the input voltage to the desired bus voltage for the second stage, and the second stage is dc/ac converter. The first stage works at a specific switching frequency, so it can be designed to achieve higher efficiency in dc/dc conversion. The second stage also works at high switching frequency and converts dc to ac commonly by using SPWM which changes the duty cycle ratio in a sinusoidal pattern. The single-stage inverter only has one stage working in high frequency while the second stage works at twice line frequency. The first stage converts dc to rectified ac waveform and the second stage unfolds it to ac. The topology of LLC resonant converter being applied for the first stage of the single-stage inverter has been proposed. This topology uses variable-frequency-modulation (VFM) which varying switching frequency on the primary side to output different voltage levels. And it achieves zero-voltage-switching (ZVS). However, LLC converter can hardly output very low voltage due to the flat voltage gain curve at high frequency. Also, LLC converter only transfers the fundamental harmonic component to the load. If the higher-order harmonic components help transfer power when the switching frequency equals the resonant frequency, the current shape will be more like a square wave and the peak of resonant current can be reduced. This thesis proposes a topology that has two L-C resonant branches in parallel for the resonant tank in the converter. And the paralleled resonant branches produce a zero-gain frequency point into the gain curve so that the gain range is enlarged within the reduced switching frequency range and 3rd harmonic component of the resonant current helps to transfer power so that the rms value of resonant current can also be reduced.

The LLC resonant converter has been widely used as a DC-DC converter due to its high efficiency, high power density and hold-up capability in power supplies for communication systems, computers and consumer electronics. Use of the high-frequency LLC converter has also been increasing in recent years due to its high power density and integrated magnetics, which reduce the total cost. With the fast development of wideband gap devices and novel magnetic materials, the trend of pushing switching frequency higher continues. However, the control characteristics of the LLC resonant converter are much more complex than that of the PWM converter due to the dynamics of the resonant tank. This paper employs state-trajectory analysis to describe and analyze the behavior of the resonant tank. Control methods based on state-trajectory analysis were used to solve the challenges in the control of the LLC resonant converter, including unpredictable dynamics, burst mode for light-load efficiency, soft start-up and short circuit protection. Additionally, digital controllers are gradually taking the place of analog controllers in the control of the LLC resonant converter due to the advantages of the digital controllers over the analog controllers, such as their ability to be flexible and re-configurable, capable of non-linear control, and able to communicate with other controllers. Among the digital controllers, cost-effective microcontrollers (MCU) are preferred for industrial applications. Because of the advantages of the state-trajectory control and the industrial preference in the cost-effective digital controllers, it would be of great benefit to apply state-trajectory control to high-frequency LLC converters with cost-effective digital controllers. This thesis investigates the impact of digital delay on state-trajectory control. Simplified Optimal Trajectory Control (SOTC) for LLC converters is further simplified so that SOTC can be achieved with cost-effective digital controllers. Furthermore, the limitations caused by digital controller are explained in detail, and methods are proposed to apply the SOTC to high frequency LLC converter is proposed. A detailed analysis of fast load transient response, soft start-up, burst mode for light-load efficiency and synchronous rectification (SR) driving is provided. Multi-step SOTC for fast load transient response is proposed to apply cost-effective digital controllers to high-frequency LLC converters; SOTC for soft start-up with only sensing Vo is proposed to minimized the impact of digital delay on state-trajectory control; SOTC for burst mode with multi-step is proposed to eliminate the limitation of minimum off-time caused by digital controllers in constant burst-on time control; a generalized adaptive SR driving method using the ripple counter concept is proposed to significantly reduce controller resource utilization for the SR control of high-frequency LLC converters. The whole control system is demonstrated on a 500kHz 1kW 400V/12V LLC converter with a 60MHz MCU, which integrates all the proposed control methods. / Master of Science

This work presents a new wireless Visible Light Communication lighting system targeted to future Smart Buildings. A digitally controlled LLC resonant dc-dc converter targeted to white LED luminaires is demonstrated. Visible Light Communication is implemented with minimal incremental cost, by operating the LLC converter in burst mode, without causing any visible disturbance. The converter operates with a regulated average LED current by adjusting the switching frequency, while the burst pulse timing is controlled to minimize the current disturbance and minimize the value of the output capacitor. Variable Pulse Position Modulation is used to modulate the data, while supporting a range of dimming settings. A digital demodulation scheme that supports variable frequency transmission is demonstrated. The 80 W, 400 V to 23 V converter experimental prototype has a peak efficiency of 93.8 %. The bit error rate of the complete system is fully characterized versus distance and angle.

This work presents a new wireless Visible Light Communication lighting system targeted to future Smart Buildings. A digitally controlled LLC resonant dc-dc converter targeted to white LED luminaires is demonstrated. Visible Light Communication is implemented with minimal incremental cost, by operating the LLC converter in burst mode, without causing any visible disturbance. The converter operates with a regulated average LED current by adjusting the switching frequency, while the burst pulse timing is controlled to minimize the current disturbance and minimize the value of the output capacitor. Variable Pulse Position Modulation is used to modulate the data, while supporting a range of dimming settings. A digital demodulation scheme that supports variable frequency transmission is demonstrated. The 80 W, 400 V to 23 V converter experimental prototype has a peak efficiency of 93.8 %. The bit error rate of the complete system is fully characterized versus distance and angle.

The low-voltage dc-dc converter (LDC) in electrical vehicles (EVs) is to convert high dc voltage (270V~430V) from traction battery to low dc voltage (12.5V~15.5V) for the vehicle auxiliary systems. Galvanic isolation is required in the LDC due to safety considerations. Three challenges exist in the LDC design: (1) wide regulation range; (2) high output current; (3) thermal management. The single stage solutions, such as phase-shift full-bridge converter and LLC resonant converter, have been widely studied in the past. A matrix transformer is widely adopted in single-stage LDC design to deal with the large current. At last, the low-profile design allows large footprint area for high power density and ease of cooling design. However, the trade-off between wide regulation range and efficiency exists in single-stage LDC design. Recently, a two-stage solution is proposed to achieve high efficiency and wide regulation range at the same time. The fixed turn ratio LLC stage serves as a dc transformer (DCX) to meet the galvanic isolation requirements and PWM dc-dc stage regulates the output voltages. In this thesis, a variable ratio matrix transformer-based LLC converter is proposed for two-stage LDC efficiency improvement. The transformer secondary copper losses are reduced by taking advantage of the adaptive number of element transformers. In addition, the PWM dc-dc stage achieves better efficiency with variable intermediate bus voltage. The operation principle and design considerations are studied in this thesis. The proposed 1600W two-stage LDC prototype achieves 96.82% full load efficiency under 400V input condition which is 1.2% efficiency higher than the fixed ratio LLC based two-stage design. Last but not least, the prototype shows a comparable efficiency to the fixed ratio LLC based two-stage design even under the low input voltage (270V) condition. / M.S. / The electrical vehicle market is growing rapidly in recent years. However, the driving range is one of the bottlenecks which imperils market growth in the future. Thus, efficient power modules in electric vehicles are desired to extend the driving range. Low voltage dc-dc converter is one of the power modules in electric vehicles which is rated at several kilowatts and converts traction battery voltage for the vehicle auxiliary system, such as air conditioner, headlights, power steering and etc. In this thesis, a variable ratio matrix transformer-based LLC converter is proposed for the two-stage low-voltage dc-dc converter efficiency improvement. Consequently, the driving range of electric vehicles is further extended.

Isolated inverters feature the freedom in voltage step-up/down, electrical safety, and modularity. Among them, pseudo-dc-link inverters have the advantage of high efficiency due to their single-stage structure. Traditionally, pseudo-dc-link inverters are based on pulse-width-modulated converters, which suffer from hard switching, the need for auxiliary components, and/or high current stresses. Meanwhile, the series resonant converter has been prevalent in past decades due to its simplicity and high efficiency. Therefore, it is intriguing to design a single-stage inverter based on a series resonant converter. However, there are limited papers regarding such an inverter topology. To figure out the reason, basic modulation methods proposed or implied in the literature are summarized and evaluated through circuit simulation software. It turns out each basic modulation method has at least one critical drawback in modulation range, hard switching, and/or high current stresses. Given the deficiencies in the basic modulation methods, a hybrid modulation method is proposed here. The proposed method combines variable-frequency modulation in the high-output region and short pulse-density modulation in the low-output region. In this way, all the aforementioned critical drawbacks can be greatly alleviated. The hybrid modulation method is compared to the basic modulation methods based on three design metrics: the rms value of the resonant current, the magnetic flux of the transformer, and the turn-off current. By these design metrics that directly related to power losses, the benefit of the proposed method in terms of efficiency can be explained. Moreover, a power loss model is also established to provide more insights into the inverter's efficiency performance. It helps demonstrate how the selection of resonant tank and other factors affects the power loss distribution. Also, an inverter design procedure is introduced and a prototype is built to verify the proposed modulation method. The results show that the switching losses, especially the turn-on loss, can be well suppressed, and the losses in other passive components are well restrained. This implies the proposed method is suitable for high-frequency applications. Other than efficiency, output waveform quality is also important for an inverter. However, the changing plant model makes the controller design difficult. Therefore, a third-order model established by other researchers has been adopted to identify the pole locations. In addition, a gain-varying method is proposed for the compensator to reduce the gain variance caused by different operating conditions. The experimental results show that without the gain-varying method, the inverter may have issues in slow tracking and/or instability. Finally, in some scenarios, the inverter based on a series resonant converter can be regarded as a module. A multi-modular inverter can be formed by connecting the modules in an input-parallel-output-series configuration. In this case, a technique termed sequential waveform synthesis can be applied. The proposed technique can extend the region of variable-frequency modulation and shorten the region of short pulse-density modulation. This is beneficial to efficiency based on an analysis. With more than a certain amount of modules connected, the short pulse-density modulation can even be waived, which means the multi-modular inverter can be free from turn-on loss. In summary, this dissertation focuses on developing modulation methods for inverters based on the series resonant converter. Soft-switching feature and high efficiency are the two top priorities. The analytic and experimental results are provided based on standalone applications. / Doctor of Philosophy / Inverters are an important part of a modern electric power system, as they convert dc electric power into ac electric power. In some applications, inverters with electrical insulation (isolated inverters) are preferred due to the need for engineering freedom, safety, and other reasons. However, each conventional isolated inverter has some of the following drawbacks: hard-switching in semiconductor devices, high circulating current, poor transformer utilization, and high complexity. These drawbacks limit the efficiency and compactness of an inverter system, making the system less attractive to practical applications. An inverter based on a series resonant converter seems to be a solution because the series resonant converter is known for being simple and highly-efficient. However, there has yet to be a proper modulation method for it. Therefore, the main contribution of this dissertation is to propose a hybrid modulation method. With the proposed method, the inverter can operate with high efficiency. Furthermore, the hard-switching can be well suppressed, which means a high-frequency, compact design is possible. Besides the theory of the proposed method, this dissertation also includes a power loss model, a hardware design procedure, and analytic comparisons with other methods. In addition, a digital approach to control the inverter is proposed. Without it, the output voltage waveform may be highly distorted. Finally, another sequential control strategy is proposed in this dissertation for an integrated system. The integrated system is composed of multiple inverters based on a series resonant converter. With the sequential control strategy, the overall output waveform quality of the integrated system can be improved.